I would suggest a Tormach mill. Its $6800, but sweet, and appears to cut steel like butter for a mid-sized mill. This mill uses ground ballscrews and steppers. It also has a 16"x10" cut envelope.

A less capable, but still very nice, $3000 option is a SuperX3 mill from Syil, at www.syil.cn Although it is less rigid, and less industrial like, (I am speaking only from specs and looks), it still is excellent for R/C use. I do not know about its steel capabilities, but its aluminum is probably just as good as the Tormach. This mill uses rolled ballscrews and steppers.
It has a 11.8"x6.2" cut envelope.

Another option would be the Industrial Hobbies mill, and cnc conversion. I believe this is $6900. This mill uses rolled ballscrews and servos. THis does have a large cut envolope of 30"x12".

Overall, iif you can fit your largest part in the any of these machines, they will be a good choice.

These are just opinions by look, and specs. I dont own any of them. - Zachery